This K23 proposal is designed to foster my development as a translational researcher in the field of HIV immunology. My proposal is based on a novel assay, called AmpliCot, which I have developed to measure the diversity of the T cell receptor (TCR) repertoire. Although the TCR repertoire is a crucial determinant of the immune response, only now does AmpliCot make it possible to quantitatively measure changes in the repertoire diversity of clinical samples. Under the experienced mentorship of Dr. Joseph M. McCune and Dr. Steven Deeks, and with the guidance of a team of expert consultants in clinical laboratory medicine (Dr. Timothy Hamill), clinical trial design (Dr. Jeffrey Martin), and biostatistics (Dr. Mark Segal), I will obtain the practical knowledge necessary to bring my laboratory investigations to the clinic, launching my career in translational research. HIV infection is known to deplete the body's number of CD4+ T lymphocytes, but little is known about the effect of infection on the repertoire of TCR specificities. I will use AmpliCot to address the following aims: Aim 1: To optimize the AmpliCot assay for use with clinical specimens and to define the normal reference interval for repertoire diversity in healthy adults. Aim 2: To characterize the damage HIV inflicts on TCR repertoire diversity. Aim 3: To assess the degree to which antiretroviral therapy can reconstitute TCR repertoire diversity. The proposed studies are planned in collaboration with two large ongoing cohort studies of HIV-infected individuals at San Francisco General Hospital, thereby facilitating recruitment and retention of subjects, streamlining patient visits, and aiding data analysis. This proposal is a first step towards testing the hypothesis that HIV-infected patients are vulnerable to opportunistic infections because they have lost important TCR specificities and have "holes" in their repertoires. This research could play an important role in helping doctors determine the optimal time to begin treating HIV-infected patients with antiretroviral therapy, and the AmpliCot assay could be used in the future to evaluate the ability of alternative treatments to reconstitute the immune system of HIV-infected patients. The data and skills I will obtain in the K23 award will serve as a foundation for a subsequent prospective study (to be proposed in a future R01 grant application) to investigate whether measurements of lymphocyte receptor diversity can predict risks of opportunistic infections or other clinical events in HIV-infected patients.